(Table 1) Osmium concentrations and isotopic ratios for ODP Leg 125 peridotites@en

Mantle peridotites drilled from the Izu-Bonin-Mariana forearc have unradiogenic 187Os/188Os ratios (0.1193 to 0.1273), which give Proterozoic model ages of 820 to 1230 million years ago. If these peridotites are residues from magmatism during the initiation of subduction 40 to 48 million years ago, then the mantle that melted was much more depleted in incompatible elements than the source of mid-ocean ridge basalts (MORB). This result indicates that osmium isotopes record information about ancient melting events in the convecting upper mantle not recorded by incompatible lithophile isotope tracers. Subduction zones may be a graveyard for ancient depleted mantle material, and portions of the convecting upper mantle may be less radiogenic in osmium isotopes than previously recognized.

从伊豆-小笠原-马里亚纳俯冲前弧（Izu-Bonin-Mariana forearc）钻取的地幔橄榄岩（mantle peridotites），其187Os/188Os比值（187Os/188Os ratios）呈非放射成因特征，数值范围为0.1193至0.1273，对应的元古代模式年龄（Proterozoic model ages）为8.2亿至12.3亿年。若这些橄榄岩为4000万至4800万年前俯冲作用起始阶段岩浆作用（magmatism）的残留相，则发生熔融的地幔其不相容元素亏损程度远高于洋中脊玄武岩（mid-ocean ridge basalts, MORB）的源区。这一结果表明，锇同位素（osmium isotopes）可记录对流上地幔（convecting upper mantle）中古老熔融事件的相关信息，而此类信息无法通过不相容亲石同位素示踪剂（incompatible lithophile isotope tracers）获取。俯冲带（subduction zones）或许是古老亏损地幔物质（ancient depleted mantle material）的埋藏场，且对流上地幔的部分区域其锇同位素放射成因程度可能低于此前的认知。